1. Field of the Invention
The present invention relates to a collimator lens used in an optical scanning apparatus for scanning a laser beam for recording or displaying an image, such as copier, laser printer, or the like; and, in particular, to a collimator lens for converting a divergent luminous flux emitted from a light source such as a semiconductor laser into a parallel luminous flux, and an optical scanning apparatus using the same.
2. Description of the Prior Art
Various kinds of optical scanning apparatus for scanning a laser beam in order to record or display an image, such as copier, laser printer, and the like, have conventionally been known.
Such an optical scanning apparatus is configured so that a laser beam emitted from a semiconductor laser is converted into a parallel luminous flux by a collimator lens, the resulting parallel luminous flux is deflected as a rotary polygon mirror rotates, and thus deflected luminous flux is focused onto an imaging surface with the aid of an f.theta. lens.
Since typically employed collimator lenses are required to mainly satisfy axial performances, many of them have been known to be constituted by two or three lens elements as disclosed in Japanese Unexamined Patent Publication Nos. 61-173214 and 61-147225, for example.
Their out-of-axis performances, however, can only be appropriate within a very narrow range in which a sine condition is satisfied. In particular, in the case employing a multi-beam system (such as one chip with a plurality of light sources in particular) intended for simultaneously recording a plurality of different items of information by one scan or the like, it is necessary for the image surface curvature to be held within the range of 10 to 20 microns while the half angle of view .omega. is about 2 degrees, thus making it difficult to employ the collimator lenses disclosed in the above-mentioned publications.
On the other hand, as a collimator lens having a larger number of lenses, the one constituted by four to six lens elements disclosed in Japanese Unexamined Patent Publication No. 61-173215 has been known.
The lens disclosed in this publication, however, is a so-called retro-focus type lens in which a negative lens is disposed closest to the parallel luminous flux, and does not have a configuration for actively reducing the image surface curvature. In practice, the lenses shown in its examples do not yield values of image surface curvature amount which are sufficient for overcoming the problem mentioned above.